The objectives of this research are twofold: to explain the relation of synaptic structure to synaptic strength, and to discover the developmental mechanisms that determine the relative strength of different synapses on a common postsynaptic neuron. Each of the sensory SR neurons in the crayfish abdomen, twenty neurons in all, synapse with an identified pair of target neurons in the last abdominal ganglion. The strengths of these synapses are graded in adult animals; posterior SR neurons make stronger synapses that anterior ones. The Specific Aims of this proposal are to describe quantitatively the structural basic of these physiological differences in synaptic strength, and to discover when and by what mechanisms these gradients of synaptic strength first appear during the animal's development. The postsynaptic response (PSP) of each of the pair of the target neurons to stimulation of each SR axon will be measured, and then a selected presynaptic SR axon and both target neurons will be filled with an intracellular marker, HRP. The synaptic contacts between the SR axon and each target neuron will be counted and mapped in the light microscope, and the structure of each contact examined in the electron microscope. By comparing the structures and locations of strong synapse with those of weak synapses, we will test three hypotheses that consider both presynaptic and postsynaptic factors: --that SR axons with strong synapses make more synaptic contacts with the target than do weak synapses. --that differential postsynaptic attenuation of PSPs arising at different sites on the target neurons causes the measured differences in synaptic strength. It is not known when these gradients of synaptic strength first arise. To discover whether this occurs during embryogenesis or during postembryonic growth, SR PSPs from the same target neurons will be measured in newly hatched crayfish. --If the gradients arise during postembryonic growth, the roles of synaptic competition among SR axons, of differences in their rates of addition of new synaptic contacts, and of growth of the targets themselves during normal development will be tested.